A mobile communication system that is made up from a Node-B (base station), an RNC (Radio Network Controller), and a CN (Core Network) is representative of an existing 3GPP (3rd Generation Partnership Project) mobile communication system.
AMR (Adaptive Multi-Rate) can be offered as the voice data encoding method in a mobile communication system of this configuration. AMR is a method in which the rate of voice data is altered dynamically according to line conditions.
In a mobile communication system that employs AMR, a transcoder is established in the CN in order to make the rates of encoding and decoding of voice data identical, and transcoding is carried out by the transcoder as necessary.
Data frames of voice data that are encoded by AMR, are made up from a plurality of subframes having different data sizes. The combination of this plurality of subframes differs according to the rate of the voice data, and an RFCI (RAB sub-Flow Combination Indicator) value is prescribed as an identifier for each of these combinations. In other words, an RFCI value is defined for each rate of voice data.
In a mobile communication system that employs AMR, RFCI information is set as control information in voice encoding in each Node-B. RFCI information includes information that identifies for each RFCI value the structure of the data frame that is indicated by that RFCI value, and more specifically, information relating to the number of subflows that make up that data frame and the data size for each sub-flow. In addition to AMR, RFCI information is also used in Wide-Band AMR (wide-band voice codec) and CS streaming services (such as Fax or modem communication).
When transmitting voice data that have been encoded at a particular rate, a Node-B transmits voice data to which the RFCI value that corresponds to this rate is appended to another Node-B, and upon receiving voice data from another Node-B, decodes this voice data at the rate that corresponds to the RFCI value that is appended to the voice data.
When UE (User Equipment: a terminal) carries out voice communication by way of two Node-Bs, if the RFCI information between the two Node-Bs matches, each RFCI value in these two items of RFCI information indicates a data frame of the same structure. As a result, encoding/decoding of voice data can be carried out between the two Node-Bs at the same rate without passing by way of a transcoder in the CN. A mode of carrying out voice communication without passing by way of a transcoder in this way is referred to as “Transcoder-Free Operation (TrFO).” This mode is prescribed in 3GPP TS23.153 (Non-Patent Document 1).
If, on the other hand, the RFCI information between the two Node-Bs does not match, data frames of different structure may be indicated in these two items of RFCI information even though the RFCI values are the same. In such cases, encoding/decoding of voice data cannot be carried out at the same rate between the two Node-Bs unless carried out by way of a transcoder, whereby voice communication cannot be carried out while maintaining transcoder-free operation (TrFO).
Accordingly, RFCI information between two Node-Bs is preferably caused to match in order to implement voice communication while maintaining transcoder-free operation (TrFO).
However, movement of UE occurs frequently in a mobile communication system, whereby RFCI information of the movement-origin Node-B to which the UE is connected before movement is believed to often fail to match the RFCI information of the movement-destination Node-B to which the UE is connected after movement.
3GPP TS25.415 (Non-Patent Document 2) prescribes the handover of RFCI information by means of a Iu-UP Initialization message that is prescribed in Iu-UP (Iu interface user plane) protocol by way of CN between the RNC in the event of SRNS (Serving Radio Network Subsystem) relocation in which the RNC to which the movement-origin Node-B of a UP is connected differs from the RNC to which the movement-destination Node-B is connected.